ASRJC 2025 H2 Chem Hydroxy Compounds Notes

2025 JC2 H2 Hydroxy Compounds 2025/ASRJC/Chemistry 1 Anderson Serangoon Junior College JC2 H2 Chemistry HYDROXY COMPOUNDS Contents/Outline 1 Introduction 2 Nomenclature and Structure 3 Physical properties of alcohols and phenols (SDL) 4 Preparation of alcohols - Electrophilic addition - Nucleophilic substitution - Reduction 5 Chemical properties and reactions of alcohols 6 Acidity of alcohols and phenols 7 Chemical properties and reactions of phenols Learning Outcomes Candidates should be able to: (a) recall the chemistry of alcohols, exemplified by ethanol: (i) combustion (ii) nucleophilic substitution to give halogenoalkanes (iii) reaction with sodium (iv) oxidation to carbonyl compounds and carboxylic acids (v) dehydration to alkenes (b) suggest characteristic distinguishing reactions for the different classes of alcohols (primary, secondary and tertiary alcohols), e.g. mild oxidation (c) deduce the presence of a CH 3CH(OH)– group in an alcohol from its reaction with alkaline aqueous iodine to form tri–iodomethane (d) recall the chemistry of phenol, as exemplified by the following reactions: (i) with bases (ii) with sodium (iii) nitration of, and bromination of, the aromatic ring (e) explain the relative acidities of water, phenol and ethanol in aqueous medium (interpret as Bronsted–Lowry acids) References 1. Organic Chemistry (Sixth Edition), McMurry, Brooks/Cole 2. Chemistry for Advanced Level, Cann and Hughes, Murray

2025 JC2 H2 Hydroxy Compounds 2025/ASRJC/Chemistry 2 1 INTRODUCTION Hydroxy compounds are organic compounds containing the functional group –OH. There are two types of hydroxy compounds: Alcohols Phenols –OH group bonded to a saturated C (sp3 hybridised) of an alkyl group –OH group bonded directly to a C (sp2 hybridised) in the benzene ring Examples: Examples: CH3CH2OH ethanol OH phenol CH2OH phenylmethanol H3C OH 4–methylphenol OH cyclohexanol NO2 OHO2N O2N 2,4,6-trinitrophenol 2 NOMENCLATURE AND STRUCTURE • Alcohols are named with the suffix, –ol, added to the corresponding alkane. CH3OH C CH H H O H H H methanol ethanol • A number is used to indicate the position of the –OH group if there is more than one possible position on the parent chain. CH3CH2CH2CH2OH CH3CHCH2CH3 OH CH3 C H OH C OH H CH2CH3 butan–1–ol butan–2–ol penta–2,3–diol

2025 JC2 H2 Hydroxy Compounds 2025/ASRJC/Chemistry 3 • The benzene ring is named as a ‘phenyl’ group when it is considered as a substituent. CH2OH CH2CH2OH CHCH3 OH phenylmethanol 2–phenylethanol 1–phenylethanol • Compounds that have a hydroxyl group directly attached to a benzene ring are called phenols. The carbon bearing the –OH group is numbered 1, other substituents are numbered relative to it. OH CH3 HO Cl OH NO2NO2 2–methylphenol 4–chlorophenol 3,5–dinitrophenol • In cases where there are higher priority substituent groups such as the carboxyl group, the –OH group is named as hydroxy in the prefix. 4–hydroxybenzoic acid • Alcohols are classified according to the number of carbon atoms attached to the carbon bonded to the –OH group. They can be primary, secondary or tertiary. Primary (1o) Secondary (2o) Tertiary (3o) methanol 1 R group 2 R groups 3 R groups C H H OHH C H H OHR C R' H OHR Note: the groups R, R’ and R’’ may or may not be the same. C R' R" OHR COOHHO

2025 JC2 H2 Hydroxy Compounds 2025/ASRJC/Chemistry 4 3 PHYSICAL PROPERTIES OF ALCOHOLS AND PHENOLS (SDL) Study this part of the topic on your own by recalling and applying Chemical Bonding concepts here. 3.1 Boiling points 1. Alcohols have considerably higher boiling points than the alkanes of comparable molecular masses. Example Compound Mr Boiling point / oC CH3CH2OH 46.0 78.5 CH3CH2CH3 44.0 –42.1 Chemical bonding concept: relative strength of hydrogen bonding vs id–id More energy is required to overcome the stronger hydrogen bonding between CH3CH2OH molecules as compared to the weaker instantaneous dipole –induced dipole attractions between CH3CH2CH3 molecules. 2. Boiling point of alcohols increases with increasing length of alkyl chain. Example Compound CH3OH CH3CH2OH CH3CH2CH2OH Boiling point / oC 65 78 97 Chemical bonding concept: Factors affecting strength of id-id: no of e− As the length of the alkyl chain increases, the number of electrons / electron cloud size increases, which results in a greater ease of distortion of the electron cloud . More energy is required to overcome the stronger instantaneous dipole –induced dipole attractions between the alcohol molecules. R O H R OH OH R + – – + – + hydrogen bonding

2025 JC2 H2 Hydroxy Compounds 2025/ASRJC/Chemistry 5 3. Alcohols with branched carbon chains generally have lower boiling points than their unbranched chain isomers. Chemical bonding concept: Factors affecting strength of id-id: surface area Example CH3 C OH CH3 CH3 CH3 C CH2OH CH3 H CH3CH2CH2CH2OH 83 °C 108 °C 118 °C Among the three alcohols, butan-1-ol is the most elongated molecule which provides the greatest surface area of contact. More energy is required to overcome the stronger instantaneous dipole–induced dipole attractions between butan-1-ol molecules. 4. Boiling point of alcohols increases with increasing number of hydroxyl groups. Chemical bonding concept: Average number of hydrogen bonding per molecule (extensiveness) Example OH OH OH OH OHOH Boiling point 137oC 195oC 290oC Mr 88.0 90.0 92.0 As the number of OH groups increases, the average no. of hydrogen bonds per molecule increases. More energy is required to overcome the more extensive hydrogen bonding between molecules and hence boiling point increases. 5. Phenols exist as crystalline solids with a low melting point of 40.5 C. As a liquid, phenol has relatively high boiling point as compared to other organic compounds of the same molecular masses. This is because a larger amount of energy is needed to overcome the stronger hydrogen bonds between phenol molecules. 6. 2-nitrophenol has a lower boiling point than 4-nitrophenol. OH NO2 OH O2N 217 oC 245 oC There is intramolecular hydrogen bonding due to the close proximity of the −OH group and the −NO2 group. This reduces the nu mber of sites available for intermolecular hydrogen bonding . Less energy is needed to overcome the less extensive intermolecular hydrogen bonding in 2-nitrophenol during boiling.

2025 JC2 H2 Hydroxy Compounds 2025/ASRJC/Chemistry 6 3.2 Solubility 1. The lower alcohols (methanol, ethanol, and propanol) are completely miscible with water. The energy released during the formation of hydrogen bonding between alcohol molecules and water molecules is able to compensate for the energy taken in to break the hydrogen bonding between alcohol molecules and the hydrogen bonding between water molecules. Example Hydrogen bonding between ethanol and water 2. The solubility of alcohols in water decreases as the number of carbon atoms in the molecule increases. Alcohols with more than 4 carbons are partially or immiscible with water (you may well end up with two layers in your test tube). As the length of the non –polar alkyl chain increases, id–id attractions between the alcohol molecules become more significant. C H H H C H H C H H C H H C H H C H H C H H C H H C H H OH Hence, the energy evolved during the formation of hydrogen bonding between the water molecules and the alcohol molecules is not enough to compensate for the energy required to break the hydrogen bonding and the stronger id-id attractions between alcohol molecules and the hydrogen bonding between water molecules. 3.